This invention relates to fiber-forming processes in which fibers are passed through a chamber for operations such as drawing, orienting and attenuation.
In many fiber-forming processes, filamentary material extruded from a die is directed through a processing chamber where, for example, the filamentary material is drawn, oriented and/or reduced in diameter. Such a processing or attenuation chamber is commonly used in spun-bond processes (see U.S. Pat. Nos. 3,502,763; 3,692,618; 4,064,605; 4,217,387; 4,812,112; 4,820,459; 5,270,107; 5,292,239; 5,571,537; 5,648,041; and 5,688,468). But it also can be used in other processes, such as meltblowing processes (see U.S. Pat. Nos. 4,622,259 and 4,988,560), meltspinning of filaments and filament yarns (see U.S. Pat. No. 4,202,855), and flashspinning of plexifilamentary film-fibril material.
Use of a processing chamber places restrictions on the whole fiber-forming processxe2x80x94restrictions intended to assure that fibers will travel through the chamber effectively without, for example, plugging the chamber. Such restrictions include limits on the speed of the fibers as they move through the chamber; limits on the configuration of the chamber to allow threading of fibers through the chamber and rethreading upon breakage of fibers; and limits on the degree to which the extruded fibers are molten or liquid as they enter the chamber.
Various efforts have been made to improve processing chambers and reduce the restrictions they impose on the fiber-forming process. One proposal is to use a wide-throated entry for the chamber, and form the chamber with a movable wall that is moved into place after polymer flow begins and which may be moved out of place if plugging occurs; see U.S. Pat. Nos. 4,405,297 and 4,340,563 as well as 4,627,811. Alternatively, U.S. Pat. No. 6,136,245 proposes beginning the fiber-forming process slowly and with a processing chamber spaced further from the extrusion die than the intended operating distance; the process is then gradually accelerated and the processing chamber moved closer to the die until it is eventually in the operating position.
In a different effort intended to achieve uniform fiber velocity across the width of an attenuation chamber, the walls of the chamber are made of a flexible material, and a grid of pressure sensors is used to activate local changes in the geometry of the wall to attempt to equalize the pressure through the chamber width; see U.S. Pat. No. 5,599,488. U.S. Pat. No. 4,300,876 describes a blower structure having only one wall curved to provide a Coanda air stream within which extruded filaments are entrained.
All of these approaches continue to leave important restrictions imposed on fiber-forming processes by use of a processing chamber.
The present invention provides a new fiber-forming method that not only alleviates many of the limitations imposed by use of a processing chamber, but more than that, greatly expands fiber-forming and fibrous-web-forming opportunities. In this new fiber-forming method, extruded filaments of fiber-forming material are directed through a processing chamber that is defined by two parallel walls, at least one of which is instantaneously movable toward and away from the other wall; preferably both walls are instantaneously movable toward and away from one another. By xe2x80x9cinstantaneously movablexe2x80x9d it is meant that the movement occurs quickly enough that the fiber-forming process is essentially uninterrupted; e.g., there is no need to stop the process and re-start it. If, for example, a nonwoven web is being collected, collection of the web can continue without stopping the collector, and a substantially uniform web is collected.
The wall(s) can be moved by a variety of movement means. In one embodiment the at least one movable wall is resiliently biased toward the other wall; and a biasing force is selected that establishes a dynamic equilibrium between the fluid pressure within the chamber and the biasing force. Thus, the wall can move away from the other wall in response to increases in pressure within the chamber, but it is quickly returned to the equilibrium position by the biasing force upon resumption of the original pressure within the chamber. If extruded filamentary material sticks or accumulates on the walls to cause an increased pressure in the chamber, at least one wall rapidly moves away from the other wall to release the accumulated extrudate, whereupon the pressure is quickly reduced, and the movable wall returns to its original position. Although some brief change in the operating parameters of the process may occur during the movement of the wall(s), no stoppage of the process occurs, but instead fibers continue to be formed and collected.
In a different embodiment of the invention the movement means is an oscillator that rapidly oscillates the wall(s) between its original position defining the chamber space, and a second position further from the other wall. Oscillation occurs rapidly, causing essentially no interruption of the fiber-forming process, and any extrudate accumulated in the processing chamber that could plug the chamber is regularly released by the spreading apart of the wall(s).
In general, a new fiber-forming method of the invention comprises a) extruding filaments of fiber-forming material; b) directing the filaments through a processing chamber defined by two parallel walls, at least one of the walls being instantaneously movable toward and away from the other wall and being subject to movement means for causing instantaneous movement during passage of filaments; and c) collecting the processed filaments.
A processing chamber having an instantaneously movable wall as described makes possible great changes in the fiber-forming process. Procedures and parameters that were previously not useful because of the danger of plugging of the processing chamber now become possible. Fiber velocities, polymer flow rates, and degrees to which the polymer is molten or liquid upon entering the processing chamber can be varied to create improved as well as essentially new processes. The invention is especially useful to enhance processes of direct web formation, i.e., processes in which fiber-forming material is directly converted into nonwoven web form, without separate formation of fibers that are then assembled into a web in a different process.
The invention also provides and makes use of a new apparatus, which briefly summarized, comprises a) an extrusion head for extruding filaments of fiber-forming material through orifices in a die, b) a chamber aligned to receive the extruded filaments for passage through the chamber, the chamber being defined by two parallel walls, at least one of the walls being instantaneously movable toward and away from the other wall; and c) movement means for moving the at least one wall, e.g., resiliently biasing the wall toward the other wall or oscillating the wall toward and away from the other wall. Movement of the wall toward and away from the other wall is sufficiently easy as to allow the described rapid or instantaneous movement, e.g., the wall will move away from the other wall in response to increases in pressure within the chamber but will be quickly returned to its original position by the biasing means upon resumption of the original pressure within the chamber; or the oscillating means will rapidly oscillate the wall between closer and further spacings.
The present invention also provides new products. For example, as discussed in more detail later in this specification, collected masses of fibers from a fiber-forming process of the invention may include fibers that are interrupted along their length, e.g., by a fiber break or entanglement. The fiber segment where the interruption occurs may differ from the main portion of the fiber in important properties, e.g., in morphological characteristics that are manifested as differences in melting point, cold-crystallization temperature, glass transition temperature, crystallinity index (indicating the proportion of the fiber that is crystalline), or crystalline type. These differences can be detected by differential scanning calorimetry or X-ray scattering. Collected masses of fibers as described are a consequence of the beneficial new fiber-forming process of the invention; and in addition, the new webs offer beneficial properties themselves. One such useful product of the invention comprises a coherent mass of fibers in web form, the mass of fibers including fibers randomly interrupted along their length by segments that are fiber-like, and are less than 300 micrometers in diameter but larger in diameter than the main portion of the fiber.